When two babies are born to the same mother at the same time, they are twins. Identical twins look almost exactly alike because they share the same genetic material, while fraternal twins might look as different as any other siblings. Being a twin means you share your birthday with someone forever, and many twins describe feeling a special connection to each other throughout their lives.

The word also describes things that come in matching pairs or are almost identical. A hotel might have a room with twin beds, meaning two separate beds of the same size. Cities across a river from each other are sometimes called twin cities, like Minneapolis and St. Paul in Minnesota. Engineers designing an airplane might create twin engines for safety and balance.

You might also call something a twin when it closely resembles something else. If your science project looks remarkably like your friend's, a teacher might joke that they're twins. When something mirrors or matches another thing, especially in an unexpected way, people sometimes say two things are twins to highlight the similarity.